Simian immunodeficiency viruses (SIV) cause a reproducible disease course in rhesus macaques identical in many aspects to HIV-1 infection in human patients, including an acute phase characterized by high levels of viremia, reduction of viremia coincident with the onset of adaptive immune responses, a prolonged asymptomatic phase leading to a gradual decline in CD4+ T-cells and culminating in increased viral load, and the development of opportunistic infections. SIVmac239 is a molecularly cloned, pathogenic isolate of SIV. We and others have found that SIVmac239 is extremely resistant to antibody-mediated neutralization, either by SIV-positive animal plasma or monoclonal antibodies directed against the viral envelope proteins, a property it shares with clinically relevant, primary isolates of HIV-1. Because of this extreme neutralization-resistant phenotype and the availability of a well-established animal model for AIDS pathogenesis, SIVmac239 is perfectly suited for a molecular-genetic exploration of the relationship between neutralization-resistance and the overall role of the antibody response in vivo (which may be considerably more complex). The four specific aims that constitute this proposal are unified under the goal of using the SIVmac239 system to marry in vitro investigation of the neutralization-resistant phenotype of primary viral isolates to a powerful model for addressing the in vivo influence of antibodies on viral replication and pathogenesis. The first specific aim addresses specfic hypotheses regarding the mechanisms by which antibodies neutralize and the corresponding viral mechanisms for evading neutralization, using genetically modified variants of SIVmac239 and quantitative assays for comparing neutralization and binding of antibodies to virions. The second part of this proposal seeks to generate antibodies with specificity for fully assembled envelope spikes. The third specific aim details experiments that seek to distinguish the contributions of antibody-mediated neutralization and antibody-dependent cellular effector mechanisms to the antiviral immune response in vivo. Finally, we propose to generate an SIV variant bearing the epitope of the broadly-neutralizing anti-HIV-1 monoclonal antibody 2F5. SIV/HIV epitope chimeric virions will be used to elucidate the elusive structural contributions of the envelope complex to the immunogenicity the 2F5 binding site.